In the conventional martensitic heat resistant alloy, the content of B is generally controlled to be in a range of: 0.008% by weight or less and the content of N is generally controlled to be in a range of 0.02 to 0.06% by weight. The content of N is set in the above-mentioned range because: N is an element which is naturally mixed into the alloy from raw steel or atmosphere and the element is inevitably contained in the alloy by the content of 0.02% by weight or so, regardless of any attempt to remove it; the presence of N in the alloy results in precipitation of carbonitrides of Nb and V, thereby enhancing the creep strength of the alloy; when the content of N is added in such large quantity as exceeds 0.1% by weight, the creep rupture ductility, welding property and workability are deteriorated; and the presence of N in the alloy is rather preferable as long as the content thereof does not exceed 0.06% by weight.
With regards to B, B contained in the alloy has an effect of minutely dispersing precipitates and suppressing the growth thereof, thereby stabilizing grain boundaries. Therefore, addition of B by a very small content significantly enhances the creep rupture strength. However, as B exhibits a strong affinity with N, addition of B by a large content results in the precipitation of itself as BN, whereby the effects, by B and N, of improving the characteristics of the alloy are all lost, and the welding property and workability of the alloy are significantly deteriorated. Due to this, in the conventional, the content of B in the alloy is reduced to an extremely small value of 0.008% by weight or less, in consideration of the content of N.
On the other hand, Japanese Patent Application Laid-Open (JP-A) Nos. 6-10041, 8-218154, 8-22583 and 9-122971 disclose a ferritic heat resistant steel or a martensitic heat resistant steel and welding materials thereof, to which a relatively large amount of B has been added. However, in any of these references, the content of B has still to be limited due to the reason described above, and the B content thereof remains insufficient with respect to the N content. Thus, the effect of drastically enhancing the creep rupture strength by adding B, which effect would be obtained if the B were to be added by a sufficient content and work without being disturbed by N, were not achieved yet.
JP 8-294793A discloses a welding material for a ferritic steel containing Al of specific type, a relatively large amount of B and a small amount of N. However, the workability and the like of the material disclosed in JP 8-294793A are not fully satisfactory. Further, the material does not achieve a sufficiently high creep strength in a range of relatively long rupture time at a high temperature.
Further, in recent years, JP 11-12693A has proposed an attempt to maximize the effect of addition of B by decreasing the content of N as much as possible. However, in JP 11-12693A, the added amount of B is still insufficient with respect to the added amount of N and the characteristic-improving effect by 9 is not fully exhibited. Thus, high creep strength in a range of relatively long rupture time at a high temperature cannot be achieved.
The present invention has been contrived in consideration of the problems as described above. On object of the present invention is to provide a martensitic heat resistant alloy which solves the problems of the prior art, maximizes the characteristic-improving effect by the presence of B of a large content, has high creep rupture strength in a range of relatively long rupture time at a high temperature, has excellent oxidation resistance, hot workability and ductility. The present invention also aims at providing a method for producing such a martensitic heat resistant alloy.